The image formation process by electrophotography involves forming an electrostatic latent image on a photosensitive body made of an inorganic or organic material, developing therefrom a toner image using a toner, transferring the toner image to paper or plastic film, and fixing the image thereon to provide a visible image. The photosensitive body is negatively or positively chargeable depending on its mechanism. When a printing character portion is left as an electrostatic latent image after exposure to light, a toner chargeable with electricity having the opposite polarity is used for developing a toner image. On the other hand, when the reversal development is performed by depriving a printing character portion of electricity while keeping the rest charged with electricity of one polarity after exposure to light, a toner chargeable with electricity of the same polarity is used for developing a toner image.
The toner contains a binding resin, a coloring agent, and other additives. Generally, the toner is further supplemented with a charge controlling agent so that it may be conferred therewith desired charging properties (charging speed, charge level, charge stability, etc.), storage stability, endurance to the change of environment, and the like. The performance of a toner may be greatly improved through the addition of such a charge controlling agent.
Suitable charge controlling agents previously proposed and actually employed for improving the negative charge controllability of a toner include mono azo metal complex compounds where the central metal is chromium, mono azo metal complex compounds where the central metal is iron, and metal complexes of alkylsalicylic acid or of aromatic oxycarboxylic acid, or their salts.
However, these charge controlling agents have the following problems. For example, they are not sufficiently affinitive to a binding resin of a toner, do not have a sufficient power to provide tribocharge, or do not allow the toner to be charged in a sufficiently short time (rate of initial increase in charging), and accordingly, images obtained at the initial phase of copying are short of clearness, or the quality of images is apt to fluctuate during continuous copying. Furthermore, if the charge controlling agent is composed of a metal complex or salt of alkylsalicylic acid or aromatic oxycarboxylic acid, the toner incorporating such a charge controlling agent greatly alters its charging performance depending on environmental conditions. Thus, the quality of images varies significantly depending on which time of the year they are copied.
Indeed there are some among mono azo chromium complex compounds that are sufficiently improved to be free from part of the aforementioned defects affecting the performance of toner. However, even such compounds, when they are burnt for disposal, may have the risk of producing hexavalent chromium or a harmful substance which may have adverse effects on the environment and human body, although the production amount of such toxic substance is likely to be minute. A toner incorporating a charge controlling agent containing a complex whose central metal is iron, instead of a complex whose central metal is chromium, has been disclosed (for example, see the following patent document 1). The toner incorporating such a metal complex has a sufficiently high charge level to be practical (at least −10 μc/g), but its initial increase in charging is lower than that of a corresponding toner incorporating a chromium complex. In addition, the lowering of charge level observed when the toner is exposed to a highly moist environment remains still unsolved.
Some azo iron complexes have also been disclosed (for example, see the following patent document 2). Of the complexes, those that ensure high performance are all azo complexes comprising two or more nitro groups. Thus, synthesis of such complex compounds is exposed at all times to the danger of ignition/explosion. The risk of ignition/explosion is particularly increased during the synthesis of complexes whose central metal is iron, and drying and pulverizing steps are extremely dangerous. Pulverized toners are generally produced by putting the material into an extrusion kneader, kneading the material, and pulverizing the kneaded material. Thus, it is not always unlikely that the powder explodes during the manufacture of the toner. As far as complexes whose central metal is chromium are concerned, the likeliness of ignition/explosion is low. Of them, however, the majority of azo chromium complexes comprising two or more nitro groups are self-reactive (Class V dangerous substances).
An iron complex having a pyrazolone skeleton is disclosed (for example, see the following patent document 3), although its application is directed towards the production of an optical information recording medium. Another iron complex having a pyrazolone skeleton is disclosed (for example, see the following patent document 4) which is used as an additive to toner. These pyrazolone compounds generally include compounds in which a phenyl group attached to a nitrogen atom of a pyrazolone ring has two or more substituent groups. However, the above patent documents do not give any mention or suggestion, in concrete terms, about a pyrazolone compound in which a phenyl group comprises two chlorine atoms at specified positions.    Patent Document 1: JP Kokai No. Sho 61-155464    Patent Document 2: JP Kohyo No. Hei 8-500912    Patent Document 3: JP Kokai No. Hei 11-20317    Patent Document 4: JP Kokai No. 2004-86224